It is usually desired in chemical reactions to quantify the result of the reaction by measuring suitable variables or to follow the progress of the reaction by means of such a variable. A well-known example is following the changes in color of a colored indicator which has been added to a reaction mixture.
In a process for amplifying nucleic acids, in contrast to usual chemical reactions, a precursor is not converted into a completely different product, but one part of a molecule is copied with the aid of other precursors. In the specific case of methods for amplifying nucleic acids, especially the polymerase chain reaction (PCR), the result of the reaction is nowadays frequently quantified by staining with ethidium bromide and detecting the fluorescent signal. It has also been proposed to detect the hybridization process itself by fluorescence. Such a method is disclosed in U.S. Pat. No. 6,174,670. The detected fluorescent signal results from a fluorescence resonance energy transfer (FRET) between two fluorophores on excitation of the donor fluorophore with light. The DNA can be genotyped by analyzing specific melting and solidification curves.
Monitoring the progress of a reaction by fluorescence is relatively complicated, because a sensitive optical system must be provided for detection. In addition, such a method is costly to use because of the fluorescence markers necessary therefor.
It is also known to examine the progress of viscosity-altering reactions, e.g. polymerization reactions, by means of viscometry. A reaction is viscosity-altering if the viscosity of the product of the mixture of the products differs from the viscosity of the mixture of the precursors. The viscosity of the mixture will then approach the viscosity of the products as the reaction increasingly advances. Measurement of viscosity then provides a measure of the advancing of the reaction. The viscometers employed for this purpose are, however, ordinarily optimized for industrial production plants and unsuitable for examining small reaction volumes.